The present invention relates to a slip rate control device for a vehicle, and more particularly, to a slip rate control device for controlling a driving wheel during the starting or acceleration of the vehicle.
In general, when the traction force of a driving wheel exceeds the friction force between the wheel and a road surface [(the coefficient of friction between a wheel and a road surface).times.(load on a driving wheel by the vehicular weight (vehicular load))] during starting or acceleration of the vehicle, the driving wheel slips. When it is assumed that the circumferential speed of the driving wheel is Vw and the vehicular speed is V, the slip rate which indicates the degree of slippage, can be obtained from the following equation: EQU .lambda.=(Vw-V)/Vw (I)
Depending on this slip rate .lambda., the friction force between the wheel and the road surface (that is, the critical value of the traction force of the driving wheel) varies as shown in FIG. 4. The friction force represented on the vertical axis reaches its maximum value at a predetermined value .lambda..sub.O. Here, the friction force between the wheel and the road surface, indicated by the solid curve, is in the travelling direction (longitudinal direction) of the vehicle, but the friction force in the transverse direction (lateral force) falls as the slip rate .lambda. gets larger, as indicated by the dotted curve in the drawing.
Based on the above consideration, in order to maximize the driving efficiency of the vehicle by maximizing the friction force in the longitudinal direction between the wheel and the road surface and also to prevent the sideslip of the vehicle by suppressing to the utmost the reduction of the friction force in the transverse direction between the wheel and the road surface, a method is known in which the slip rate .lambda. is detected and controlled so that it will be close to the predetermined value .lambda..sub.O. More specifically, for example, a lower limit value .lambda..sub.1 and an upper limit value .lambda..sub.2 of a predetermined range including said predetermined value .lambda..sub.0 are set for the slip rate .lambda. in accordance with the vehicular speed V. Then, the circumferential speed V.sub.W of the driving wheel is controlled by controlling a torque control unit according to the value of the slip rate .lambda. obtained from the driving wheel speed Vw and the vehicular speed V, thereby performing feedback control of the slip rate .lambda. of the driving wheel so that it falls within the above predetermined range .lambda..sub.1 -.lambda..sub.2.
However, when the gear ratio of the transmission is changed, even if the vehicular speed is not changed, the driving force of the driving wheel is changed, thus the variation width and the variation rate (variation quantity) of the slip rate .lambda., which are control parameters, are also varied. For example, the variation width and the variation rate of the slip rate .lambda. become larger when the gear of the transmission is set low than those when the gear is set high, thus causing a problem in that the control accuracy of the slip rate .lambda. is diminished.